A drug company has been cleared to test experimental drugs on intellectually disabled people, despite earlier concerns that it was illegal and risky.

It comes amid renewed debate over medical trials conducted on patients without their permission or even knowledge, with the Ministry of Health recently cautioning its Health and Disability Ethics committee to adhere to law after "concerns" were raised about some approved trials.

Drug company Roche Products sought approval to test the safety of a new drug, RG1662, on adolescents and young adults with Down syndrome, which it hoped would increase their IQ.

The initial application included testing on adults in Wellington, Dunedin and Auckland who were not mentally capable of giving consent and had no parents to consent on their behalf.

In April, the ethics committee declined the drug trial. It said it was unconvinced the drug would benefit participants, many of whom could not understand it.

The drug was yet to be proven safe and the potential side effects included increased risk of suicide.

"It was not clear that the proposed research would be in the best interests of the participants," the committee said.

However last week, the committee agreed to approve the trial to proceed with children only, with their parents' consent.

Auckland University professor Ed Mitchell, who is running the study in New Zealand, said despite the approval the restrictions imposed were still "hugely problematic" and meant people who could benefit from new drugs were missing out.

No other country had declined to approve the trial for adults and there were wider implications for other groups who could not provide consent, such as emergency or intensive care patients.

"The interpretation of the law has changed and needs to be reviewed," he said.

But Lynda Williams, of the Auckland Women's Health Council, said the balance was still tilted against patients' rights and she believed many non-consensual medical trials were illegal.

The trials were mostly about drugs companies trying to grab market share rather than benefiting patients, she said.

"But it's not just about whether we are guinea pigs in some drug trial. It's the whole practice of not seeking consent and that being OK."

Medical researchers usually needed a patient's consent or, if the person was unable, the consent of a family member. Ethical approval is sometimes granted for studies where it is not possible to seek consent, such as for people in a coma, if the research would benefit the patient and they would probably consent.